this is totally normal, he has only been on this earth for 4 months, he is probably scared and needs you right now, Some babies are different and need more attention, espesially since he has the back rash, reflux and allergies, he needs his mommy more than ever. Our society is weird, every other country the mother is very close to their babies, even holding them in a sling all day and sleeping with them at night, don't let our society let you tell you how you should be a mother, you know best what he needs, sleep with him if he needs you, you don't have to sleep in the same bed but beside the bed in a co-sleeper. Also for the reflux keep him laying on his back because it's safer but go buy a little mattress pad that will keep him about 30 percent in an upright position, you could try adding some blankets underneath his upper back and head for now.
If he is breastfeeding then breastfed babies get hungry very fast and don't tend to sleep as much as formula fed babies, keep breastfeeding if you are as much as possible your milk is natural and the best thing to give him. He is also probably going through a growth spurt, which will make him want to eat more and sleep less. This will not last forever, just be there for him, if you need some sleep, try breastfeeding him and when he is done then feed him about 2 oz of formula, but don't do this too often or your breast supply may go to low.
I slept with my daughter until she was almost 6 months old, and I didn't care what anybody else said and their opinions, she needed her mother and it made her feel safe and secure and independent and at 9 months she was sleeping in her own crib in her own room and slept for 12 hours a night, now she is in her own toddler bed, a child grows independent and feels loved when they know you will always be there for them, hope this helped. Babies and toddlers go through stages, so if he starts sleeping well for a couple of months then stops sleeping so well, that is also normal it's just somthing us parents have to deal with. We have our issues and problems and they have theres we just have to understand and respect that, (and pray that we can nap anytime we can) LOL
also, get a ritual going now, he may not notice what you are doing but it will prepare you and he will catch on eventually, make up somthing to do before you put him to bed, what I did with my daughter was give her a warm bath, put lotion on her and put some warm pj's on and then hold her and sing to her while I walked around the house and turned off all the lights and when bedtime was near I whispered in a soft voice, then cuddled with her in our bed, when she was still awake I would gently carry her over to her sleeping place, this has worked like a charm (not every night) but about 80 percent of the time which is still awsome.
we also tried a bouncy seat next to our bed and a swing next to our bed, the point is to try everything, it's survival at this point you need your sleep , he will be ok